Abstract
1. 1. The strength of cooperativity of rabbit muscle glycogen phosphorylase b (α-1,4-glucan:orthophosphate glucosyltransferase, EC 2.4.1.1) has been investigated with the aid of Hill plots using glucose 1-phosphate as the varying substrate and glucose 6-phosphate as the allosteric inhibitor. The homotropic cooperativity of glucose 1-phosphate sites has been found to be completely abolished on treatment of the enzyme with 1-fluoro-2,4-dinitrobenzene in the presence of AMP (1 mM) and orthophosphate (10 mM) until 20% inactivation occurred. 2. 2. Desensitization does not occur on dinitrophenylation until 50–60% inactivation in the presence of glucose 1-phosphate or AMP or both. Also, the presence of either AMP or orthophosphate in the reaction mixture has been ineffective in desensitising the enzyme. Samples inactivated up to 80% in the absence of any ligand retain partial allosteric character. 3. 3. The desensitization is assignable to modification of one lysyl and one cysteinyl residue. The role of orthophosphate in exposing some specific area (or groups) in the enzyme is discussed.
Published Version
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